I knew the US presidential race was over last week when my son preemptively announced that he had lost his bet with me: Hillary Clinton was not going to be the Democratic candidate. The question of whether Barack Obama can beat John McCain is still open, according to the opinion polls, but it probably won’t stay open long once the two men go head to head. McCain has many attractive qualities, but he is 71 and Obama is 46.

McCain is also a Republican in a year when the US is heading into a recession after eight years of a Republican administration. Even more importantly, he is committed to continuing a war in Iraq that most Americans just want to leave behind. Curiously, this means that the two men with the greatest potential influence on McCain’s political future are Osama bin Laden and Moqtada al-Sadr.

The one thing that could swing the 2008 election in favour of the Republicans is another large-scale terrorist attack on the United States. If al-Qaeda has any ability to provide that attack, it will certainly do so, for Osama bin Laden is well aware that his greatest recruiting tool in the Arab world is the American military presence in Iraq. But it is unlikely that al-Qaeda has any significant presence within the United States.

Moqtada al-Sadr is a more interesting case. He is the leader of the Mahdi army, the biggest Shia militia in Iraq, and he has just extended his unilateral ceasefire against American troops and rival militias for another six months. His two main objectives in life are to evict the US from Iraq and to gain control of the Iraqi government, and the first is a necessary preliminary to the second.

So long as the US presidential election promises to result in an administration pledged to withdraw from Iraq, he doesn’t have to lift a finger. But if by August it looks like McCain has a chance of winning, then Moqtada al-Sadr has every incentive to end his ceasefire and launch a mini-Tet offensive against US troops. The point would not be to win. It would be to remind American voters that Iraq is a quagmire that they should leave really soon.

So one way or another, Barack Obama is almost certain to be the president of the United States by January of next year. He has hedged his commitment to withdraw American troops from Iraq in various ways from time to time, but there is little doubt in most people’s minds that he really intends to do it. What will the Middle East look like after the Americans are gone?

Not just gone from Iraq, either. There are currently US military bases of one sort or another in almost every country along the south-western (Arab) side of the Gulf, but with Iran emerging as the new great power of the region, many of the host countries will soon be asking the Americans to leave. They don’t fear invasion by Iran; they fear internal destabilisation if Iran incites their own Shia minorities against them. So keep Tehran happy by sending the Americans home.

Iraq, contrary to all the predictions of disaster, will probably be all right after the withdrawal of US troops. It will never again be the secular, female-friendly society of the past, and it will take at least a decade to recover from the economic devastation of the embargo, the invasion and the occupation, but it won’t break up.

Most of the smaller ethnic and religious minorities have fled from Iraq or been killed, and the larger groups – Sunni Arabs, Shia Arabs, Kurds – have mostly retreated into homogeneous districts and neighbourhoods, so there’s not much left to fight about except along the boundary between Arab Iraq and Kurdistan. It’s even possible that the more or less democratic system imposed by the US occupation will survive the departure of the Americans.

Iran will indeed emerge as the new paramount power of the Gulf, but its actual influence even over predominantly Shia Iraq will be quite limited. Farther afield, the notion of a dangerously radical "Shia crescent" running through Iraq, Syria and Lebanon is sheer nonsense: Shias are a minority in Lebanon, and a very small minority in Syria. It is mainly the US State Department that promotes this fantasy, with the aim of scaring Sunni Arab states into a new, US-dominated alliance against Iran.

The real fall-out from the US invasion of Iraq is the greatly heightened prestige of Islamist revolutionaries throughout the Arab world. Whether this will ever result in a successful Islamist revolution in a major Arab country remains to be seen – they have been trying and failing for thirty years now – but the odds have probably shifted somewhat in that direction.

And the big loser of this decade’s events is Israel, which must now deal with a strengthened Iran, a Gaza Strip under Islamist control, and a United States in retreat from the Middle East. It still faces no serious military threat from its neighbours, but its political options are significantly narrower than they were.